From the center of the circle, draw a radius (len = 2.5cm) to point D. Now you get a right angled triangle with the sides : x cm, 1.5 cm and 2.5 cm. Using Pythogoras, you get x^2 = 2.5^2 - 1.5^2 which gives x = 2 cm.

Let us consider that the centre of the circle lies between A and Let it be O
Then join O and D
In the triangle DOB
OB =(2.5-1)
Do=radius=2.5cm
Therefore applying Pythagoras theorem
X^2+1.5^2=2.5^2
Solving we get x= 2cm